Stainless steel forms a stable passive film in nitric acid thereby exhibiting excellent corrosion resistance. However, high-temperature and concentrated nitric acid, for example, a temperature of 80 to 90° C. and a concentration of 90% by mass, has an extremely strong oxidizing power and causes transpassive corrosion in general stainless steels. Further, transpassive corrosion facilitates general corrosion in whole, which involves dissolution of Cr2O3 which forms a passive film.
An example of materials having corrosion resistance in this type of environment includes high-Si austenitic stainless steels disclosed by Patent Documents 1 and 2. These high-Si austenitic stainless steels have excellent nitric acid corrosion resistance due to formation of a silicate (SiO2) film in a transpassive region.
However, regarding acid resistance, although no serious problem has occurred, there are cases where corrosion is excessively facilitated, the causes of which are unclear in many respects, and a solution for such cases is needed.
Moreover, in a high-Si austenitic stainless steel, because of a concentration of Si, a large amount of inclusions and intermetallic compounds are formed in steel, causing deterioration of hot workability. To solve this problem, Patent Document 3 discloses that hot workability is improved by limiting the chemical composition such that Al is 0.05% or less (“%” regarding chemical composition means “mass %” unless otherwise stated in the present description) and O is 0.003% or less, and by eliminating formed intermetallic compounds through hot rolling after performing soaking and/or temperature uniformity at 1100 to 1250° C. for long hours. The inclusions are limited in the total amount, and not limited in their types.
Although Patent Document 4 discloses defining an amount of sol. Al to prevent the production of oxides which deteriorate corrosion resistance in working-flow, it has given no consideration on inclusions produced in molten steel, and is silent on the deterioration of corrosion resistance caused by inclusions. Since in general, the amount of inclusions such as Al2O3 is not directly related to the amount of sol. Al, simply controlling the amount of sol. Al is not enough to prevent problems caused by inclusions.
Patent Document 5 discloses that corrosion resistance is improved by finely dispersing inclusions based on the idea that inclusions originally occurs corrosion. However, it only facilities fine dispersion of MnS by controlling the amount of S and hot rolling conditions, and discloses nothing on alumina inclusions and others.
Patent Document 6 discloses an invention to prevent pitting corrosion by making a cluster of inclusions granular to make the inclusions water insoluble through the control of the composition of the inclusions. However, such inclusions hinder the formation of a silicate film which is needed to improve corrosion resistance under high-temperature and concentrated nitric acid.